External Factors

Several factors external to the plant, originating in its immediate environment, can each independently limit the rate of photosynthesis. Light is the sole energy source driving both the photo-oxidation of water and the excitation of pigment molecules, so photosynthetic rate rises directly with light intensity, though the exact intensity at which further increases stop helping (the light-saturation point) differs by species: heliophytes, or sun-loving plants such as bean, need a higher light intensity to reach their maximum rate than sciophytes, or shade-loving plants such as Oxalis. The duration (quantity) of light exposure matters too, since plants exposed to light for longer periods (long-day plants) generally achieve higher photosynthetic rates. The quality (wavelength) of light matters as well, because the photosynthetic pigment system does not absorb every wavelength equally: the biologically usable band, called Photosynthetically Active Radiation (PAR), spans roughly 400 to 700 nm, with red light producing the highest photosynthetic rate of any wavelength and green light producing the lowest, consistent with chlorophyll's own absorption spectrum (section 13.6.1). Carbon dioxide, though present at only around 0.03% (roughly 330 ppm) of the atmosphere, plays an outsized role: increasing its concentration increases photosynthetic rate, but only up to a point - beyond roughly 500 ppm, further increases stop helping and can even become mildly inhibitory. Oxygen has the opposite relationship: rising oxygen concentration actually decreases photosynthetic rate, an inhibitory effect first discovered by Warburg in 1920 using the green alga Chlorella, and mechanistically explained by RuBisCO's oxygenase activity (photorespiration) competing directly with its productive carboxylase activity as oxygen levels rise. Temperature has an optimum that varies considerably by species - commonly falling somewhere between 25 and 35 degrees Celsius, though some specially adapted plants differ widely from this, such as Opuntia (around 55 degrees Celsius), lichens (around 20 degrees Celsius), and algae living in hot springs (up to 75 degrees Celsius); temperatures that are either too high or too low both tend to close stomata and inactivate the enzymes photosynthesis depends on. Water contributes both directly, as the electron and proton source split during photolysis, and indirectly, by controlling stomatal opening and the hydration state of the protoplasm; water stress specifically reduces the supply of NADPH+H+ available to the plant. Minerals affect photosynthesis at several distinct steps: magnesium, iron and nitrogen are needed for chlorophyll synthesis; phosphorus is needed for phosphorylation reactions; manganese and chloride are needed for the photolysis of water; and copper is needed to form plastocyanin. Air pollutants such as sulphur dioxide, nitrogen dioxide, ozone and general smog all measurably depress the rate of photosynthesis. Two classic, simple apparatus experiments are traditionally used to study these effects directly: Willmott's bubbler co …